The heater has increased quick responsiveness to temperature control of air, and the air conditioner can realize highly accurate temperature control using the heater. A duct unit having heat sources comprising electric lamps in the inside thereof is attached to an air duct. Air coming into contact with glass bulbs of the electric lamps is heated by radiant heat and transferred heat generated by lighting the electric lamps, whereby the heater with increased quick responsiveness to temperature control is obtained. A first heater and a second heater are provided in the air duct, respectively, and temperature sensors for controlling these heaters are attached to the air duct and a precision environment chamber, respectively, whereby temperature control in two stages with different temperature resolutions is made possible, and an air conditioner with high accuracy excellent in controllability can be provided.